This invention relates to a method and system for testing cable (CATV) systems, and more particularly, to a method and system for a frequency sweep test instrument for testing the frequency response of a CATV system.
Cable televisions systems, often called CATV systems, have many miles of coaxial cable strung throughout their service area with amplifiers and other elements interspersed throughout. The CATV system has a number of channels over which are transmitted television signals. Each channel has a unique frequency. The subscribers to the CATV system are connected to the coaxial cable system.
To ensure the quality of the signals being transmitted over the coaxial cable system, the coaxial cable system must be periodically checked. Also, as is the case with any system having the complexity of cable systems, cable systems suffer periodic problems and failures. Often, such problems and failures results from the failure or degradation of an individual component of the cable system, such as an amplifier, and only a portion of the cable system is affected. When this happens, the individual component causing the problem must be located so that it can be repaired or replaced.
One way in which coaxial cable systems are checked is by testing their frequency response over the various channels. Heretofore, this has been accomplished by injecting a signal into the cable system at its front end and measuring the signal level at locations remote from the head end. Since cable systems have the capacity for carrying several channels and the frequency response of the coaxial cable may vary with frequency, it is necessary to check the cable system by measuring its frequency response for each channel that can be carried by the cable system. This is accomplished by sweeping the channels that can be carded by the coaxial cable system by sequentially injecting a signal into the cable system at each channel frequency at the cable systems' head end and measuring the signal level of each test signal at the remote location or locations.
To frequency sweep test a cable system, two pieces of test equipment are typically used, a head end test unit and a remote test unit. The head end test unit is connected to the cable system at its head end. The remote test unit is connected to the coaxial cable at a desired location remote from the head end. The head end unit runs the test by sending telemetry signals to the remote test unit followed by the frequency sweep test signals. The telemetry signals sync the remote test unit to the head end test unit and both units then sweep the cable system, the head end unit injecting test signals at each frequency and the remote unit measuring signal strength. At the end of the sweep, the head end test unit then again sends telemetry signals to the remote test unit. In addition to syncing the remote test unit to the head end test unit for the next sweep, the telemetry signals also contain information indicating the frequencies to be swept, and the voltage level at which the test signals will be injected. The remote test unit receives these results and based on its own measurements and the injection level indicated in the telemetry message, determines the frequency response of the cable system. The remote test unit typically displays this frequency response.
A problem with this heretofore used frequency sweep testing technique is that it disrupts service to the subscribers. When the sweep is being performed, the test signal being injected at each channel by the head end test unit will interfere with the subscriber's receptions of that channel. Although this interruption is brief, it nevertheless causes a noticeable interruption in the subscriber's reception of that channel. Moreover, when conducting the frequency sweep test, the cable system is usually swept several times.
The head end test unit heretofore used have required two transmitter sections. One transmitter section is used to generate and transmit the telemetry signals to the remote test unit and the other transmitter section is used to generate and transmit the test signals.
It is an object of this invention to provide a frequency sweep test method and system that eliminates the need to disrupt service when conducting the sweep test.
It is also an object of this invention to reduce the cost and complexity of the head end test unit by replacing the two transmitters in the head end test unit with one.